THE OLD WORLD MONKEYS 61 absent altogether ; and the permanent placenta is diffuse and non-deciduate. In Tarsius and the Anthropoidea there is not even a transient yolk-sac placenta ; and the permanent placenta forms a single or double disc. In Tarsius and in the Simiidse and Man there is a single disc, but the Monkeys have frequently two. Tarsius is distinguished from the Apes and Monkeys in the characters of the villi on the placenta, for its villi are not very distinct and are covered by syncytium only. In the Monkeys and Apes the villi are very distinct, and are capped by a syncytium and a cellular layer of Langhans. In the Apes and Man the villi are more deeply embedded in the maternal tissues than in Tarsius.